I can add these devices manually without problems. So, I think the community is configured correctly.

In addition, all devices have same community but the discovery only discover some devices.

Can not the problem be other?

Adding a device manually does not mean the community string is correct. Other than Discovery, DCR does not contact the device.

Other than incorrect SNMP community strings, access-lists or firewalls, it could be a timeout issue. If these devices take longer 12 seconds to reply to SNMP, then they would be marked as unreachable.

The easiest thing to do is start a sniffer trace filtering on traffic to one of these unreachable devices. Then, start a new Discovery. When it completes, look at the trace to see what packets were sent, and what credentials were used.

Thanks for reply, I will try to start a sniffer.

Other question about this, if I add a device manually, does not the community string mean is correct? I don't know because after I add a device manually and without I introduce device type for example, later the device appears in DCR with device type corresponding. Is not this data collected by SNMP? If it is not collected by SNMP, how is it collected?